The ALFALFA HI Absorption Pilot Survey: A Wide-Area Blind Damped Lyman Alpha System Survey of the Local Universe
Jeremy Darling, Erin P. Macdonald, Martha P. Haynes, and Riccardo, Giovanelli

TL;DR
This pilot survey of the local universe using the ALFALFA survey searched for HI 21 cm absorption lines, finding no intervening systems but setting upper limits that align with previous optical and emission studies, and demonstrating the potential of future surveys.
Contribution
First wide-area blind HI absorption survey in the local universe, establishing sensitivity limits and upper bounds on HI column density distribution, paving the way for future large-scale absorption studies.
Findings
No intervening absorption systems detected.
Upper limits consistent with previous optical and emission surveys.
Detection rate aligns with existing observations.
Abstract
We present the results of a pilot survey for neutral hydrogen (HI) 21 cm absorption in the Arecibo Legacy Fast Arecibo L-Band Feed Array (ALFALFA) Survey. This project is a wide-area "blind" search for HI absorption in the local universe, spanning -650 km/s < cz < 17,500 km/s and covering 517.0 square degrees (7% of the full ALFALFA survey). The survey is sensitive to HI absorption lines stronger than 7.7 mJy (8983 radio sources) and is 90% complete for lines stronger than 11.0 mJy (7296 sources). The total redshift interval sensitive to all damped Lyman alpha (DLA) systems (N_HI >= 2x10^20 cm^-2) is Delta z = 7.0 (129 objects, assuming T_s = 100 K and covering fraction unity); for super-DLAs (N_HI >= 2x10^21 cm^-2) it is Delta z= 128.2 (2353 objects). We re-detect the intrinsic HI absorption line in UGC 6081 but detect no intervening absorption line systems. We compute a 95% confidence…
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